Process for heating liquids in tube furnaces



y 1950 5. J. H. BREUKEL 2,514,535

PROCESS FUR HEATING LIQUIDS' IN TUBE FURNACES v flied June -10, 1941 Fig. 2

,kdaand a V/IA 5and5' 1 w [//I J) VA 1) VA 7} vJJUJJQ JJUu I His Afiorneq Patented July 11, 1950 PROCESS FOR HEATING LIQU IDS IN TUBE FURNACES Stephanus J. H. Breukel, The. Hague, Netherlands, assignor to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application June 10, 1947, Serial No. 753,602 In the Netherlands June 14, 1946 This invention relates to heating viscous liquids, such as petroleum and petroleum fractions in a tube furnace comprising a plurality of heating zones wherein the tubes are exposed to different degrees of heating.

Furnaces commonly comprise a radiant section and a convection section, which may be separated by a wall or may be different portions of the same chamber. The radiant section, in either case, contains the tubes which are exposed to direct radiation from the hearth, so that the transfer of heat to the tubes containing the liquid to be heated is efiected largely by radiation, some heating by contact with the combustion products usually being also effected. The convection section is the part of the furnace wherein the tubes are in some manner shielded from direct radiation from the hearth, i. e., Where radiation from the hearth does not penetrate either because of a dividing wall, or because the tubes in the radiantsection are interposed between the hearth and the convection section. The convection section is traversed by hot gases coming from the hearth, and may be in a separate enclosure from that housing the radiant section;

hence, in this space the heat is largely transferred to the tubes by convection from the gases.

In the usual type of tube furnace the tubes of the radiant section and of the convection section are connected in series, thus forming one continuous path for the liquid to be heated, although often several banks are provided, forming a plurality of parallel paths, each traversing both furnace sections. In the past difficulty has been experienced in regulating the flow of the liquid in relation to the heat transferred to the tubes in the radiant section. It is necessary to provide a sufficient velocity to lead off the heat from the tube walls to prevent damage to the tubes.

In the standard type of furnace used hereto fore in which, as was stated above, the tubes in the convection and radiant sections are connected in series, the velocity in the entire system of tubing is determined by the high velocity that must be attained in the radiant section; the velocity, therefore, will be high in all tubes, and

turbulent flow will obtain in all or in the greatest portion of the system. The resistance to flow and, hence, work required for pumping will be high as a result of the high velocity and will, in the case of highly viscous oils, often be excessive.

' According to the present invention, it was found that pumping costs can be materially reduced and even highly viscous liquids heated in furnaces comprising convection and radiant sec- 9 Claims. (Cl. 122356) tions without danger to the tubes by flowin the liquid first through the tubes in the convection section in a mainly laminary flow and then through the tubes in the radiant section under conditions of turbulent flow, by selecting appropriate velocities of flow in the respective tubes.

The tubes of the radiant section of the furnace are exposed to high temperatures and to prevent their being damaged by collapsing or burning through their inner walls must be intensively cooled by the liquid to be heated therein, and this is possible only if the liquid is in turbulent fiow. As the formulae for heat transmission indicate, when there is turbulent flow the tube wall temperature is considerably lower than when there is laminary flow.

In the convection section the heating is not so intense, i. e., the heat rating per surface unit of tubing is lower than in the radiant section and, moreover, the temperature of the liquid is lower, so that there is less danger due to overheating of the tubesandturbulent flow is not necessary. It is desirable to keep the velocities of liquid flow as low as possible in these tubes to reduce pumping costs, and laminary flow is desirable.

To create turbulent flow it is necessary to maintain the Reynolds number above a certain minimum value. The formulae for the Reynolds number are given in different forms for tubes of various shapes. One common form is:

wherein Re is the Reynolds number; '0 is the velocity of flow in feet per second; r is the hydraulic radius of the tube in feet; and n is the kinematic viscosity of the liquid in square feet per second. (See Chemical Engineers Handbook by Perry, second edition, page 799 for other formulae.) For circular tubes this takes the form:

perimentally, although in the case of most furnaces having sharp return bends in the tubing or having provisions designed to promote turbulence at the entrance to the radiant section, it is usually safe to substitute 2300'in the equations given above.

Because the liquid is further heated in the radiant section, its velocity of flow and viscosity are not constant, and theReynoldsnumber will be different for different parts, usually increasing in the direction of flow, unless tubes of different diameters are used in different portions."

Accordingly, the Reynolds numbers discussed herein are the lowest that most obtain at any part of the radiant section, usually the entrance to the radiant section.

To economize on pumping costs it is desirable to use as low a velocity as possible in the .radiant section, but above the critical velocity. From the formula given it will be noted that a particular value of Re, say 2100, can be attained at lower values of v if n, the kinematic viscosity, is lowered. This viscosity lowering is effected preferably by heating the liquid to the greatest possible extent in the convection section, before it reaches the radiant section. Keeping the velocity v in the radiant section as low as possible is desirable because the flow resistance encountered by the liquid in the tubes increases substantially with v, with'the result that the work'necessary for pumping the liquid also increases sharply. Lowering the viscosity not only permits lower velocities in the radiant section under turbulent flow conditions but also further reduces the flow resistance, which increases both'with velocity and viscosity.

;While velocities considerably above the critical 7 velocity are possible in the radiant section, Ipre- .fer, for the reasons explained above,to operate at the lowest possible flow velocities. My preferred range of flow velocities for the radiant section is that giving Reynolds numbers between 2100 and 3300; but to avoid the danger of laminary flow due to streamlining .of the tubes and to achieve greatest possible economies it is best to operate between the limits of 2300 and .2900. I do not,

however, limit myself to operations below 3300,

, largely by balancing the cost of the installation against the pumping economies, and no fixed value can be set. I prefer to make the ratio of velocities in the radiant'and convection sections between about 1 to 2 and l to 6. Velocitiesconsiderably below the critical velocity are, however, ,preferred and the flow is always laminary.

The flow velocities described above can be attained either by using tubes of appropriate diameters or by manifolding the tubes so that a number of tubes, arranged in parallel in the convection section, issue into a single tube or into a smaller number of tubes in the radiant section, using tubes of the same diameter throughout, or even using tubes of smaller diameterin the convection section. A combination of these expedients may also be employed, i. e. the tubes may be manifolded and the tubes in the radiant section may be of smaller diameter than those of th convection section.

In the drawings forming a part of this specification which illustrate one specific embodiment of the invention and is not intended to delimit the types of furnace in which the process may be applied:

Fig. l is a cross sectionalview of a furnace adapted for practicing the process; and

Fig. 2 is a sectional View, partly in side elevation, taken on line 22 of Fig. 1.

In the drawings, the furnace comprises brickwork I containing a hearth 2 at its lower part,

:heated by a number of liquid fuel burners 3. The

tubes to'be traversed by the liquid to be heated are arranged in the upper part of the furnace in horizontal parallel rows and staggered. The two bottom rows 8 and 8 and 9 and 9', which are nearest to the hearth and exposed to radiation therefrom, form the radiant section; the tubes above them form the convection section.

The oil to be heated is introduced at 4 and distributed over the eight'series of tubes which are connected in parallel. These eight tubes are connected into two groups, '5 and 5. Each of the eight series of tubes has two horizontal tubes in each layer, the pipes of each series being connected together at their ends with U-fittings. The tubes of the ninth layer (counting from the top) are 'connected to collecting manifold pipes 6 and G, so that the four series of group 5 are joined to the pipe 6 and the four series from the group? are joined to the pipe 6'. The collecting pipes 6 and '5', which can be interconnected for the purpose of equalizing the pressure, communicate with tubes of layers '1, 8 and 9, and with tubes of layers 1, 8' and 9,'respectively, which form the bottom three layers of tubes. In these bottom three layers all tubes of one group are connected in series, i. -e., the liquid from pipe 5 traverses, in successiomall of the horizontal tubes in layer 1 then all tubes in layer 8 and, finally, those in layer 9. The liquid from pipe 6' has a similar flow through the tubes .of layers 1, 8' and '9'. The heated liquid is withdrawn at It and It)".

"It will be seen that, if tubes of the same diameter are used throughout,'the velocity in 'thetubes of the lower three layers will be four times as great as that in the upper nine layers, if the expansion due to heating is ignored. The velocity of flow is, according to the present invention, selected so that there will be laminary flow in the convection tubes of the upper nine layers and turbulent flow in the tubes 8, 8, 9 and 9' of the radiant section, whereby the latter can be kept suitably cool and the work for pumping can be kept within reasonable bounds. The flow in the lowest layer of the convection section, I and 1 may be either laminary or "turbulent. If a sufficiently low flow velocity is used the oil will have laminary flow in the first part of the layer I and l, and assumes turbulent flow as its viscosity is lowered by further heating. operation results in the greatest economy, but to provide a margin of safety it is usually preferable to have turbulent flow also in the layers I and I.

The furnace can, of course, be of a type different from that shown in'the drawing; for instance, a furnace can be constructed with the radiation and convection tubes fitted in more or less separate compartments. Nor is it necessary that all of the tubes be of the same diameter, although such an arrangement is expedient from a practical viewpoint; for example, the radiation tubes may have diameters only half of the diameters of the convection tubes. It is, in that case, not

Such a 'mode of necessary to link up a munber of convection tubes to one radiation tube in order to obtain a transition from laminary flow to turbulent flow, since one radiation tube connected to one series of convection tubes results, in the former, in a velocity of flow which is four times than in the latter. While D, the diameter is halved, v, the velocity, is quadrupled, with the result that Re, the Reynolds number, is doubled, if the viscosities and specific volumes are taken as constant. See Equation 2.

It is desirable to design the tube fittings connected to the collecting tubes 6, 6' to avoid streamlining, whereby turbulent flow will occur at comparatively low Reynolds number, e. g. 2100 to 2300. This expedient may be applied at the entrance to the radiant section in any type of furnace.

This invention is intended particularly for those cases wherein the heating of the liquid does not lead to vaporization. When vaporization takes place the flow velocity and the viscosity change as a consequence of the changed state of aggregation to such an extent that transition from one type of flow to the other is possible, even without any special arrangement for reducing the effective flow area in the radiant section. However, the present invention is applicable even in furnaces wherein liquid is vaporized in those portions of the furnace wherein vaporization has not yet occurred.

The present process may be applied to advantage, for example, to the heating of heavy Venezuelan water containing crude oil to make it suitable for dehydration treatment. It may, however, be applied not only to liquids containing or consisting of petroleum fractions, but to any other liquids as well.

I claim as my invention:

1. A process for heating a liquid in a tube furnace comprising a hearth, a radiant section and a convection section, comprising the steps of flowing the liquid first through the convection section under conditions that laminary flow is produced at least in the major portion thereof, thereby partially heating it and reducing its viscosity, and then flowing the liquid through the radiant section under conditions that turbulent flow is produced while radiating heat directly from said hearth to the tubes of the radiant section.

2. The process according to claim 1 in which the velocity of flow of the liquid in the radiant section is between 2 and 6 times the velocity of flow in the convection section.

v3. The process according to claim 1 in which the conditions of flow in the convection section are such that the Reynolds number is below about 2100 and the conditions of flow in the radiant section are such that the Reynolds number at the entrance to the radiant section is between about 2100 and 3300.

4. The process according to claim 1 in which. the conditions of flow in the convection section are such that the Reynolds number is below about 2100 and the conditions of flow in the radiant section are such that the Reynolds number at the entrance to the radiant section is between 2300 and 2900.

5. A process for heating highly viscous liquids in a tube furnace comprising a hearth, a radiant section and a convection section, comprising the steps of flowing the liquid first through the convection section at a comparatively low velocity to result in laminary flow at least in the major portion thereof, thereby partially heating it and reducing its viscosity, then flowing the liquid through the radiant section at a higher velocity suflicient to insure turbulent flow therein while radiating heat directly from said hearth to the tubes of the radiant section, and withdrawing the heated liquid from the radiant section as a single liquid phase.

6. The process according to claim 5 in which the velocity of flow of the liquid in the radiant section is between 2 and 6 times the velocity of flow in the convection section.

7. The process according to claim 5 in which the velocity of flow in the convection section is such as to result in a Reynolds number below about 2100 and the velocity of flow in the radiant section is such as to result in a Reynolds number between'about 2100 and 3300 at the entrance to the radiant section.

8. The process according to claim 5 in which the velocity of flow in the convection section is REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,619,977 Huff Mar. 8, 1927 1,767,297 Lewis et al June 24, 1930 1,845,739 Black et al Feb. 16, 1932 2,075,601 Barnes Mar. 30, 1937 2,338,708

Bragg Jan. 11, 1944 

